The present invention generally relates to the field of smectic liquid crystals. More specifically, the invention relates in one aspect to an antiferroelectric liquid crystal device (AFLC device). A second aspect of the invention relates to a liquid crystal material as such, usable in such devices, especially an antiferroelectric crystal material of a new class. A third aspect of the invention relates to a liquid crystal device including a smectic anticlinic, non-chiral liquid crystal material.
The inventive AFLC device may especially be implemented as an electro-optic device, e.g. a display or a light modulator, but it can also be implemented as a passive component, e.g. a compensation film, where the unique properties of the inventive liquid crystal material also may be used.
The invention is related to the type of liquid crystals that is classified as smectic, i.e. with the molecules forming adjacent layers. In particular, the invention relates to anticlinic liquid crystals, meaning that molecules in adjacent smectic layers are tilted in opposite directions relative to the layer normal z. In the case where the material is also chiral, this so-called anticlinic order gives the liquid crystal antiferroelectric properties. In this case we speak of an antiferroelectric liquid crystal, abbreviated AFLC in the following. An AFLC has a local polarization along each smectic layer. The direction of this polarization is determined by the tilt direction to be either in the +y or −y direction. The anticlinic order therefore also corresponds to antipolar order which is the characteristic ground state for an antiferroelectric material.
The applications of the invention may find the most important examples in AFLC displays (AFLCDs). The AFLC device principle looks very attractive for high-resolution displays, and very important industrial investments have been made in order to realize such displays.
Considering that the present invention is thus presumably of high interest for use in AFLC Displays (AFLCDs)—although this is by no means the only field of use of the invention—a short description of a conventional AFLCD structure and the operation thereof will now be given with reference to FIGS. 1 to 4. In connection therewith, the problems encountered with such prior-art AFLCD structures will also be discussed in order to give a better understanding of the technical background of the invention.
FIGS. 1 and 2 schematically illustrate a small section of a conventional AFLCD structure, in which an antiferroelectric liquid crystal material (AFLC) 10 is confined between two solid supports 12 and 14, usually glass plates, although other materials may also be used. The two substrates 12 and 14 are separated in a controlled way by spacers (one spacer is very schematically illustrated at reference numeral 16) leaving space for the AFLC material 10. As schematically illustrated in FIGS. 1 to 3, the molecules 18 of the AFLC material 10 are arranged in parallel smectic layers 20.
The surfaces of the substrates 12, 14 facing the AFLC material 10 are coated with suitable electrode layers 22, 24 for defining pixels and suitable molecule aligning layers 26, 28. Further, the cell (substrates 12, 14+AFLC material 10) is orientated in such a way between two crossed polarizers 30, 32 that the smectic layers 20 and the layer normal z are essentially parallel and perpendicular, respectively, to the transmitting directions 30′ and 32′ of the polarizers 30 and 32. In use, electric fields E will be applied to the AFLC material 10 by means of the electrode layers 26, 28. In color displays, a color filter 29 would also be included.
As schematically illustrated in FIG. 3b, in a bulk sample of an AFLC material and without any external field applied (E=0) the molecules 18 would be tilted in essentially opposite directions (anticlinic order) with respect to the layer normal z in adjacent smectic layers 20. In a conventional AFLC bulk sample, the molecules 18 would not only be in an anticlinic order, but also form a helical superstructure, i.e. a helix in the direction of the layer normal z, mainly due to the molecular chirality. The period or the pitch of this helix usually extends over hundreds or thousands of smectic layers, e.g. in the order of 1 micron. However, in thin cells having a cell thickness comparable to the pitch of the helix, the helical superstructure maybe suppressed by surface action. This situation is referred to as a surface-stabilized antiferroelectric liquid crystal (SSAFLC). In the illustrated example of the prior-art AFLCD structure in FIGS. 1 and 2, the AFLC material 10 is assumed to be in such a surface-stabilized (SSAFLC) state. In the following, the term SSAFLC is defined as an AFLC material presenting no overall helix structure. It should be noted that the surface-stabilization requires either that the material is pitch-compensated (infinite pitch) or at least that the helical pitch is long compared with the thickness of the cell.
The smectic layers 20 of the SSAFLC material 1 would ideally be oriented perpendicular to the confining substrates 12, 14 (bookshelf structure) and with the smectic layer normal z oriented in a unique direction parallel to the substrates However, the inventors have demonstrated that, in practice, the prior-art AFLCD structures deviate slightly from this, having chevron-shaped folds in the layers. The reason for this phenomenon will be discussed below. Thus, it should be noted that FIGS. 1 and 2 are schematic especially in the sense that the prior-art structures will not present an ideal bookshelf structure.
In the zero-field condition E=0 (FIG. 3b), the anticlinic structure of the prior-art SSAFLC material is generally biaxial with the three principal indices of refraction, nα, nβ, and nγ (with the definition nα<nβ<nγ) and with the two crystallographic optic axes lying in a common plane (the yz-plane) essentially perpendicular to the cell plane. Thus, for normal incident light in the zero-field condition (E=0), the AFLC material acts effectively as a uniaxial retarder with its effective optic axis directed parallel to the substrates 12, 14, i.e. in the cell plane, along the smectic layer normal z. The γ direction represents the effective optic axis and is along the smectic layer normal. Hence, for crossed polarizers along and perpendicular to the smectic layer normal z we would, in principle, get a dark state. However, if a sufficiently strong electric field E is applied over the cell, the liquid crystal material would be switched to a bright state.
At E>Eth and at E<−Eth (applied perpendicular to the cell plane), the AFLC material is switched from the anticlinic antiferroelectric state (AF) into one of two synclinic, ferroelectric states (±F), referred to as an AFF transition. More specifically, depending on the sign of E, the effective optic axis will be tilted away an angle +θF or −θF from the polarizer axis in the ferroelectric states, as indicated in FIG. 3a (E>+Eth) and FIG. 3c (E<Eth). This gives bright states, and as the optic axis of the two ferroelectric states is symmetric around the z direction, the two states give the same transmission.
FIG. 4 schematically illustrates a hysteresis loop for the above-described operation of the prior-art AFLCD structure. The transmission T of the cell is plotted against the applied electric voltage V over the cell. The above-mentioned threshold field values +Eth and −Eth correspond to threshold voltage values +Vth and −Vth, respectively, in FIG. 4.
Even in passive multiplex drive, the gray scale of an AFLC device can be directly controlled by the amplitude of the applied voltage V, as exemplified at transmissions T2, T3 and T4 in FIG. 4. The electrooptic characteristics of AFLCs allows for a very simple addressing of AFLC displays. In a matrix display, the individual picture elements (pixels) are formed by the overlaps between the “row electrodes” on one substrate and the “column electrodes” on the opposite substrate. The individual rows are addressed one at a time (multiplex drive).
However, in all multiplexing there is crosstalk, i.e. also pixels not belonging to the addressed row (selected row) will be subjected to non-zero voltages while driving the display. In the simplest case, the gray level of a given pixel is set by applying a writing pulse followed by a holding voltage VH. The holding voltage VH prevents the pixel from returning to the dark state after the writing pulse is no longer applied. Therefore, two conditions must be met: (i) VH must be lower than the threshold voltage Vth for AFF switching and (ii) VH must be higher than the maximum voltage for which the AFLC material would return from the ferroelectric to the antiferroelectric state FAF transition). In other words, VH lies between the up slope and the down slope of the double hysteresis loops in FIG. 4.
As a result of the above-described crosstalk, in passive-matrix AFLCDs each individual pixel is always subjected to a voltage which is at least as high as the holding voltage VH. Thus, pixels which are supposed to be completely black, will not at all experience V=0 but instead V≈VH. Due to the pretransitional effect, these pixels will not give a transmission T=0 (total extinction) but rather T=T1 as indicated in FIG. 4. The crosstalk in combination with the pretransitional effect below the AFF transition therefore leads to a severe light leakage in the “dark state” of prior-art AFLCDs.
During the last decade, considerable attention has been given to the potential use of AFLCs in high-resolution flat-panel displays and microdisplays for computers and TV. Despite the obviously very attractive and well-known electro-optic characteristics of AFLCs, such as the tri-state switching behavior, easy DC-compensation, fast response (microseconds), easy gray scale, and wide viewing angle, there are still no commercial AFLC devices available on the market.
The main reason why AFLC devices have still not become commercially viable is the relatively low contrast achieved so far. The contrast is essentially ruled by the extinction in the dark state. In the following, this problem in the prior art will be referred to as “the dark-state problem”.
In general terms, the dark state can be said to be determined by the electrooptic behavior in a voltage range centered on V=0 and limited on both sides by the holding voltage ±VH. More specifically, there are two contributions to the dark-state problem in the prior art, one static and one dynamic, giving rise to a static light leakage and a dynamic light leakage, respectively. The static light leakage relates to the poor quality (homogeneity) of the AFLC bookshelf alignment and, as a result thereof, there are spatial variations of the direction of the effective optic axis in the sample. Thus, the effective optic axis is not along the transmission direction of one polarizer and a homogeneous black state cannot be achieved. The dynamic light leakage is due to the above-mentioned pre-transitional effect, i.e. a thresholdless response below the threshold (±VTH) for the AFF transition or, equivalently expressed, from the anticlinic state to one of the synclinic states.
As the dark-state problem is directly related to the bad quality of alignment, the main effort has up to now been directed to develop and to optimize AFLC materials and polymer aligning layers, in order to improve the quality of the bookshelf alignment. However, such measures have far from solved the problems and, as will be demonstrated below, they cannot in fact solve them.
As stated above, very important industrial investments have been made in order to realize AFLC displays. In the review article A. Fukuda et al. “Antiferroelectric Chiral Smectic Liquid Crystals”, J. Mater. Chem. 4, 997-1016 (1994) one full color display prototype is presented (p 1013, plate 1) representing the state of the art at that time. Alongside with the experimental and theoretical investigation of the AFLC materials and devices, an extensive chemical development has also taken place, involving the synthesis of new materials. This materials' development can be followed, during the last decade, e.g. in the patent sequence U.S. Pat. No. 5,340,498 (Arai), U.S. Pat. No. 5,723,069 (Mineta), U.S. Pat. No. 5,728,864 (Motoyama), U.S. Pat. No. 5,968,413 (Mine) and U.S. Pat. No. 6,002,042 (Mine), priority dates ranging from 1992 to December 1997.
U.S. Pat. No. 5,340,498 (Arai, priority date 1992) shows in FIG. 2 (transmittance vs. applied voltage) a relatively narrow hysterisis loop and a very strong pre-transitional effect, i.e. a substantial change of transmission from zero voltage up to 10 volts, where the distinct AFF transition takes place. This pre-transitional transmission change represents almost 15% of the total transmission change and seriously compromises the contrast. A high contrast allowing gray scale would not only require a much broader hysterisis loop, but, in particular, that the dynamic (pre-transitional) effect is essentially zero. In U.S. Pat. No. 5,723,069 (Mineta, priority 1995), the hysterisis loop shown in FIG. 1 is much broader. However, the dynamic leakage or pre-transitional effect is still considerable. Similar hysterisis curves can be found in all other relevant publications on the subject.
Sometimes the pretransitional effect is even much stronger. For instance, in the AFLC materials described by Robinson et al., Liquid Crystals 23, 309 (1997) and Liquid Crystals 25, 301 (1998), this effect is so dominant (FIGS. 3 and 4a, respectively) that it is not possible to define even an approximate threshold value for the AFF transition. These are the same materials as also described in GB 3,317,718 (Coles) from 1999. As already pointed out above in connection with FIG. 4, an AFLC display has to be driven such that there is a holding voltage ±VH applied all time to all pixels. This means that no pixel is ever at zero voltage (V=0), but instead feels a voltage of at least VH, even when it is supposed to be in the dark state. In other words, in the prior art structures the dark state can never be better than the transmission value T at V=±VH, identified as T1 in FIG. 4. As this transmission T1, due to the pretransitional effect, is of the order of 1 to 2%, the contrast in the prior art has never been better than 50:1 or at most 100:1. In order to get an enhanced contrast of, e.g., 1000:1, this pretransitional effect would have to be reduced by about one order of magnitude, such that the transmission T1<0.1%. This would mean that the slowly rising part of T as a function of V starting at V=0 (i.e. the lower slope in FIG. 4) has to be essentially horizontal, along the V-axis, at least until the value V=±VH. The ideal case would be that this part of the curve is essential horizontal until the AFF transition occurs. In the discussion of the present invention here, we might call such an ideal behavior a “sharp” AFF transition, or say that such a transition does occur without any, or substantially without any, pretransitional effect. However, it should be noted that such a “sharp transition” has never been achieved in the prior art.
The static and dynamic leakage for prior-art AFLCDs together limit the contrast to about 100:1 when measured on a pixel in the laboratory and to about half of this, i.e. 50:1, when measured on a real prototype, under driving conditions. If one realizes that a full size, full color liquid crystal television screen (of which there is no realization today) has to have a contrast of about 500:1 in order to be acceptable, one can thus appreciate why no commercial AFLC screen has ever been manufactured, in spite of the development of a number of prototypes of increasing performance, which have been presented between 1992 and 1997. Since the last-mentioned year, the further development of prototypes based on this device idea has essentially been shut down.